Notwithstanding the specific mechanism of how the characteristics are achieved, we claim that we have demonstrated a structure and a process to fabricate the same to achieve high field effect mobilities and high current modulation in organic-inorganic hybrid perovskite-based TFTs. While the present invention has been described with respect to preferred embodiments, numerous modifications, changes, and improvements will occur to those skilled in the art without departing from the spirit and scope of the invention. All references cited herein are incorporated herein by reference and all references cited by the reference cited herein are incorporated herein by reference.
This invention pertains to the field of organic-inorganic hybrid materials as the semiconducting channels in thin film field effect transistors (TFT), and in particular to the low-voltage operation of such transistors, using high dielectric gate insulators, in applications such as flat panel displays.
Thin film field effect transistors (TFT) used in liquid crystal display (LCD) and other flat panel applications typically use amorphous silicon (a-Si:H) or polycrystalline silicon as the semiconductor and silicon dioxide and/or silicon nitride as the gate insulator. Recent developments in materials have led to the exploration of organic oligomers such as hexathiophene and its derivatives, and organic molecules such as pentacene (G. Horowitz, D. Fichou, X. Peng, Z. Xu, F. Garnier, Solid State Commun. Volume 72, pg. 381, 1989; F. Garnier, G. Horowitz, D. Fichou, U.S. Pat. No. 5,347,144) as potential low-cost and/or low-temperature replacements for amorphous silicon as the semiconductor in thin-film field-effect transistors. Field effect mobility in the range of 1 cm2 Vxe2x88x921 secxe2x88x921 has been achieved in pentacene based TFT""s with SiO2 as the gate insulator (Y. Y. Lin, D. J. Gundlach, S. F. Nelson, T. N. Jackson, IEEE Electron Device Lett. Vol. 18 pp. 606-608 1997), making them potential candidates for such applications. A major drawback of these pentacene-based organic TFTs is the high operating voltage that is required to achieve high mobility and simultaneously produce high current modulation (typically about 100 V when 0.4 xcexcm thick SiO2 gate insulator is used). Reducing the thickness of the gate insulator would improve the above mentioned characteristics but there is a limit to the decrease of the insulator thickness, which is imposed by manufacturing difficulties and reliability issues. For example in the current generation of TFT LCD devices the thickness of the TFT gate insulator is typically 0.3 to 0.4 xcexcm. Recently, it was shown that high mobility can be achieved in pentacene devices comprising a high dielectric constant (∈) gate insulator, at lower voltages than pentacene TFTs using a comparable thickness of SiO2 (C. D. Dimitrakopoulos, P. R. Duncombe, B. K. Furman, R. B. Laibowitz, D. A. Neumayer, S. Purushothaman, U.S. Pat. No. 5,981,970 and 5,946,551, C. D. Dimitrakopoulos, S. Purushothaman, J. Kymissis, A. Callegari, J. M. Shaw, Science, 283, 822-824, (1999); C. D. Dimitrakopoulos, J. Kymissis, S. Purushothaman, D. A. Neumayer, P. R. Duncombe, R. B. Laibowitz, Advanced Materials, Vol. 11, 1372-1375, (1999)).
Recently a new class of TFT was demonstrated, one that comprised an organic-inorganic hybrid material as the semiconductor such as the organic-inorganic perovskite (C6H5C2H4NH3)2SnI4. This class of organic-inorganic hybrid materials can be defined as xe2x80x9cmolecular scale compositesxe2x80x9d (K. Chondroudis, C. D. Dimitrakopoulos, C. R. Kagan, I. Kymissis, D. B. Mitzi; xe2x80x9cThin Film Field Effect Transistors With Organic-Inorganic Hybrid Materials As Semiconducting Channelsxe2x80x9d; Ser. No. 09/245,460; Filed on Feb. 5, 1999; xe2x80x9cOrganic-Inorganic Hybrid Materials as Semiconducting Channels in Thin-Film Field-Effect Transistorsxe2x80x9d, C. R. Kagan, D. B. Mitzi, C. D. Dimitrakopoulos, Science Vol. 286, 945-947, (1999).). These transistors have problems similar to the ones described above for pentacene: high operating voltage is required to achieve high mobility and simultaneously produce high current modulation (typically about 60 V when 0.5 xcexcm thick SiO2 insulator is used). Reducing the thickness of the gate insulator lowers the required operating voltages to achieve the above mentioned characteristics. Again there is a limitation to the decrease in insulator thickness, which is imposed by manufacturing constraints and reliability issues, especially for large area applications such as flat panel displays, where the gate insulator is not thermally grown on Si single crystals but is deposited on top of a gate electrode. Using a high dielectric constant gate insulator to achieve high mobility at lower voltages, as was shown in organic TFTs, is not an obvious solution since such an insulator should not be expected to have an effect on the mobility measured from a crystalline inorganic semiconductor, where mobility is considered a constant parameter. In the organic-inorganic perovskite (C6H5C2H4NH3)2SnI4) conduction takes place in the inorganic component and the organic component is insulating (D. B. Mitzi, C. A. Feild, W. T. A. Harrison, A. M. Guloy, Nature, Vol. 369, 467-469 (1994); K. Chondroudis, C. D. Dimitrakopoulos, C. R. Kagan, I. Kymissis, D. B. Mitzi; xe2x80x9cThin Film Field Effect Transistors With Organic-Inorganic Hybrid Materials As Semiconducting Channelsxe2x80x9d; Ser. No. 09/245,460, Filed on Feb. 5, 1999). While this is true for C6H5C2H4NH3)2SnI4, other hybrid materials can be designed in which the organic part consists of conjugated organic molecules, such as an oligothiophene containing molecule for example, and the inorganic part is insulating and is used to template the organization of the conjugated organic molecules, thus increasing their conductivity and/or mobility.
The electrical characteristics of TFTs having the organic-inorganic hybrid perovskite (C6H5C2H4NH3)2SnI4 as the semiconductor, a heavily doped Si-wafer as the gate electrode, 500 nm thick thermally grown SiO2 as gate insulator, and Pd source and drain electrodes, are adequately modeled by standard field effect transistor equations (S. M. Sze xe2x80x9cPhysics of Semiconductor Devicesxe2x80x9d, Wiley, New York., 1981, pg. 442), as shown previously (K. Chondroudis, C. D. Dimitrakopoulos, C. R. Kagan, I. Kymissis, D. B. Mitzi; xe2x80x9cThin Film Field Effect Transistors With Organic-Inorganic Hybrid Materials As Semiconducting Channelsxe2x80x9d; Ser. No. 09/245,460; Filed on Feb. 5, 1999; xe2x80x9cOrganic-Inorganic Hybrid Materials as Semiconducting Channels in Thin-Film Field-Effect Transistorsxe2x80x9d, C. R. Kagan, D. B. Mitzi, C. D. Dimitrakopoulos, Science Vol. 286, 945-947, (1999)). The organic-inorganic perovskite (C6H5C2H4NH3)2SnI4 used in these devices behaves as a p-type semiconductor. FIG. 1, cited from xe2x80x9cOrganic-Inorganic Hybrid Materials as Semiconducting Channels in Thin-Film Field-Effect Transistorsxe2x80x9d, C. R. Kagan, D. B. Mitzi, C. D. Dimitrakopoulos, Science Vol. 286, 945-947, (1999)), shows the dependence of the current flowing between the source and drain electrodes (ID) on the voltage applied to the drain electrode (VD), at discrete voltages applied to the gate electrode (VG). When the gate electrode is biased negatively with respect to the grounded source electrode, (C6H5C2H4NH3)2SnI4-based TFTs operate in the accumulation mode and the accumulated carriers are holes. At low VD, ID increases linearly with VD (linear region) and is approximately given by the equation:                               I          D                =                                            WC              i                        L                    ⁢                      xe2x80x83                    ⁢                      μ            ⁡                          (                                                V                  G                                -                                  V                  T                                -                                                      V                    D                                    2                                            )                                ⁢                      V            D                                              (        1        )            
where L is the channel length, W is the channel width, Ci is the capacitance per unit area of the insulating layer, VT is a threshold voltage, and xcexc is the field effect mobility. xcexc can be calculated in the linear region from the transconductance:                                           g            m                    =                                                    (                                                      ∂                                          I                      D                                                                            ∂                                          V                      G                                                                      )                                                              V                  D                                =                const                                      =                                                            WC                  i                                L                            ⁢                              xe2x80x83                            ⁢              μ              ⁢                              xe2x80x83                            ⁢                              V                D                                                    ,                            (        2        )            
by plotting ID vs. VG at a constant low VD and equating the value of the slope of this plot to gm.
When the drain electrode is more negatively biased than the gate electrode (i.e. xe2x88x92VDxe2x89xa7xe2x88x92VG), with the source electrode being grounded (i.e. VS=0), the current flowing between source and drain electrodes (ID) saturates (does not increase any further) due to the pinch-off in the accumulation layer (saturation region), and is modeled by the equation:       I    D    =                    WC        i                    2        ⁢        L              ⁢          xe2x80x83        ⁢                            μ          ⁡                      (                                          V                G                            -                              V                T                                      )                          2            .      
FIG. 2a shows the dependence of ID on VG in saturation in a semilogarithmic scale (C. R. Kagan, D. B. Mitzi, C. D. Dimitrakopoulos, Science, volume 286, 945-947, (1999)). The field effect mobility can be calculated from the slope of the {square root over (|ID+L |)} vs. VG plot. FIG. 2b shows a plot of the square root of ID vs VG. A mobility of 0.55 cm2 Vxe2x88x921 secxe2x88x921 is calculated from this plot.
The present invention demonstrates TFT structures that overcome the need to use high operating voltages in order to achieve the desirable combination of high field effect mobility and high current modulation, without having to reduce the thickness of the insulator. Such structures contain an inorganic high dielectric constant gate insulator layer (for example, barium zirconate titanate) in combination with an organic-inorganic hybrid material as the semiconductor (for example, the organic-inorganic perovskite (C6H5C2H4NH3)2SnI4).
The present invention provides methods to produce organic-inorganic hybrid TFT structures wherein the high dielectric constant gate insulator is deposited and processed at temperatures compatible with glass (below 400xc2x0 C.), plastic substrates in general (below 400xc2x0 C.), and transparent plastic substrates (below 150xc2x0 C.) which are substantially lower than the processing temperatures of these materials when they are used for memory applications (up to 650xc2x0 C). An advantage of this invention are methods to produce organic-inorganic hybrid TFT structures wherein the high dielectric constant gate insulator and all the other materials are deposited and processed at temperatures below 100xc2x0 C.
A broad aspect of the claimed invention is a transistor device structure comprising: a substrate on which an electrically conducting gate electrode is disposed; a layer of gate insulator disposed on the gate electrode; an electrically conductive source electrode and an electrically conductive drain electrode disposed on the high layer of gate insulator; and, a layer of an organic-inorganic hybrid semiconductor disposed on said gate insulator and said source electrode and said drain electrode.
Another broad aspect of the invention is a transistor device structure comprising: a drain a source electrode, a drain electrode, a gate electrode, a gate insulator and a semiconductive material disposed between and in electrical contact with said source electrode and said gate electrode, said gate insulator is disposed between said gate electrode and said active region, said semiconductive material is an organic-inorganic hybrid material.